This invention relates to the production of high purity alkyl glyoxylate. More particularly, the invention relates to the production of high purity alkyl glyoxylate from mixtures containing alkyl glyoxylate, alkyl glycolate, water, alcohol and miscellaneous impurities.
High purity alkyl glyoxylate is important as an intermediate for synthesis of other compounds, for example, alkyl glyoxylate polymers. (U.S. Pat. No. 4,144,226 issued Mar. 13, 1979 to M. M. Crutchfield et al). For its use in production of alkyl glyoxylate polymers, it is particularly important that the content of alkyl glycolate, water and alcohol be low enough to avoid excessively low average chain length in the polymer.
The prior art discloses methods for producing alkyl glyoxylate by the oxidation or dehydrogenation of alkyl glycolate to alkyl glyoxylate. See for Example U.S. Pat. No. 1,614,195 issued Jan. 11, 1927 to Alfred Haussler and U.S. Pat. No. 4,340,748 issued July 20, 1982 to Herbert Baltes et al.
These prior art methods for producing alkyl glyoxylate are conducted in the gaseous state and are represented by the following reaction (1). ##STR1##
In this reaction (1) the conversion of alkyl glycolate to alkyl glyoxylate is not complete and the product of the reaction contains in addition to the alkyl glyoxylate, unconverted alkyl glycolate, water, alcohol which is produced by side reactions or derived from the alkyl glycolate feed, and miscellaneous impurities. Following condensation and recovery of such mixtures, some or all of the contained alkyl glyoxylate is present in chemically combined forms such as alkyl glyoxylate hydrate, alkyl glyoxylate oligomers, and various hemiacetals of alkyl glyoxylate. To obtain high purity alkyl glyoxylate an additional process is required to convert the hydrate or hemiacetal to alkyl glyoxylate per se. Such conversion is made very difficult by the very complex interreactions of the components of the mixture under distillation conditions which limit both the purity of the alkyl glyoxylate and its recovery per pass. In the prior art, this conversion has been accomplished by treatment with P.sub.2 O.sub.5 followed by distillation to liberate the alkyl glyoxylate (W. Oroshnik and P. E. Spoerri, J. Amer. Chem. Soc. 1941, 63,3338). While this method is effective, it suffers several serious disadvantages. Large amounts of P.sub.2 O.sub.5 are consumed, which adds considerably to the cost of production of high purity alkyl glyoxylate. Also, alcohol liberated from the hemiacetals is consumed by reaction with the P.sub.2 O.sub.5 and thus cannot be conveniently recovered in a suitable form for reuse in the production of alkyl glycolate and/or alkyl glyoxylate. Furthermore, the reaction with P.sub.2 O.sub.5 is highly exothermic and can be difficult to control under some conditions.
It is apparent that there is a need for a more efficient method for producing high purity alkyl glyoxylate from mixtures containing alkyl glyoxylate, alkyl glycolate, water, alcohol and miscellaneous impurities. One advantage of the present invention is that it provides such a process which avoids consumption of P.sub.2 O.sub.5 or other such reagents and which recovers liberated alcohol for reuse. A further advantage of the present process is production of much higher product purity with higher conversion per pass than can be obtained using conventional distillation techniques without the use of P.sub.2 O.sub.5.